Negative birefringent films greatly enhance the image quality of liquid crystal displays viewed at off-normal angles by compensating for image degradation due to "natural" net positive birefringence of the liquid crystal materials. Typically, negative birefringent films have been prepared by precision stretching of polymer films or by precisely controlled vapor deposition of thin ceramic layers. However, practicioners have found that precise control of film stretching in order to exactly control the resulting birefringence is difficult, and that ceramic films can be unstable and are not practical for large surface areas.
Non-stretched negative birefringent polyimide films are known. U.S. Pat. No. 5,344,916 describes polyimide films prepared from benzene dianhydrides and/or bis(benzene)dianhydrides and monoaromatic and/or polyaromatic diamines. While the polyimides are generally soluble, some polyimides are soluble only in solvents that are incompatible with certain useful liquid crystal polymeric substrates onto which the polyimides could be coated. The monoaromatic and/or polyaromatic diamines are linked benzeneamine compounds (benzidine derivatives, bis(4-aminophenyl)methane derivatives, 2,2-bis(4-aminophenyl)propane derivatives, and the like) rather than polyaromatic diamines such as 9,9-bis(4-aminophenyl)fluorenes. Many linked benzeneamines must be handled with care due to their suspected human toxicity.
Homopolymers of 9,9-bis(4-aminophenyl)fluorenes and aromatic dicarboxylic acid dianhydrides are known. U.S. Pat. No. 4,897,092 describes the preparation and properties of polyimide membranes prepared from the homopolymers, the membranes being useful for gas separations. The membranes are described as being approximately ten times the thickness of layers useful in liquid crystal displays, and optical properties of the membranes are not elucidated. No copolymers of 9,9-bis(4-aminophenyl)fluorenes and aromatic dicarboxylic acid dianhydrides with a non-fluorenyl aromatic diamines are described. French patent application number 2,650,829 discloses a number of possible uses for homopolymers of 9,9-bis(4-aminophenyl)fluorenes and aromatic dicarboxylic acid dianhydrides, including alignment films for liquid crystal displays. No copolymers with additional non-fluorenyl aromatic diamines are described, and no indication of optical compensation properties of the films is presented. Copolymers of 9,9-bis(4-aminophenyl)fluorenes with two or more aromatic dicarboxylic acid dianhydrides are described in U.S. Pat. No. 4,845,185 and in Japanese patent application number 62292836 (Derwent abstract). Polyimides described in U.S. Pat. No. 4,845,185 are shown to be solvent soluble and to have high heat resistance, but are only moderately light transmissive. Copolymers of 9,9-bis(4-aminophenyl)fluorene, an aromatic tetracarboxylic acid dianhydride and an aliphatic diamine are disclosed in Japanese patent application number JP 63295633. Polyimides described therein are said to have good thermal resistance and workability, and find use as electric or electronic materials (Derwent abstract), but no optical proper ties are described.
Japanese patent application 5-31341 discloses a polyimide gas separation membrane which is a copolymer of 9,9-bis(4-aminophenyl)fluorene and aromatic dicarboxylic acid dianhydride with a non-fluorenyl polycyclic fused ring aromatic diamine. No optical properties of these copolymers are disclosed.